Antigen recognition by T cells

Question 1

Define antigen

Answer 1

Proteins, carbohydrates and lipids capable of binding BCRs, TCRs and/ or innate immune receptors to induce an immune response

Question 2

What is an epitope?

Answer 2

• Specific portion of an antigen, that a complementary Ab will bind to
• Each antigen will have multiple epitopes that different Abs can bind to = polyclonal response
• Ags will be degraded and processed, and the epitope will be presented on MHC to T cells

Question 3

Why cant T cells recognise native antigens?

Answer 3

• Native = not processed
• B-cells can be actiavted if their IgM is cross-linked, allowing proliferation and production of Abs
• Native Antigen has to be processed and presented on MHC before T cells can produce a response

Question 4

What immune cells recognise and process ag?

Answer 4

• APCs
• Monocytes = have CD14+ (dont express huge amounts, but they form a reservoir for differentiation into macrophages and DCs)
• Macrophages = CD11b+
• DCs = CD11c+ (myeloid) or CD123+ (plasmacytoid)
• B cells = CD19+ (BCR)

Question 5

What are macrophages and DCs?

Answer 5

• Rarely found in peripheral blood - in mucosal tissues
• High phagocytic cells - induce strong T-cell responses and inflammation
• Macrophages = better equipped to kill pathogens (higher NO production)
• DCs = better at migrating to lymph nodes (via CCR7) and presenting ag to T-cells

Question 6

What do B cells do?

Answer 6

• Highly abundant in blood and mucosal tissue
• Receptor-mediated internalisation of Ags as opposed to phagocytosis
• Differentiate into plasma cell to make Ab
• Good APCs
• Main inducer of T-cell response against pathogens

Question 7

Fixed vs viable APC

Answer 7

• If an APC is viable, it can be presented with native protein, process it and present the ag on MHC to T cells
• If an APC is fixed, it is metabolically inert and so can only present the ag if it has been pre-digested

Question 8

What are the 2 pathways of antigen presenting?

Answer 8

• Exogenous - ag taken up from outside the cell. EC bacteria e.g. N.meningitidis or s.aureus
• Endogenous - ag from inside the cell. Viruses e.g. HIV, tumour antigens

Question 9

What are the different processes of exogenous antigen uptake?

Answer 9

• B cells use receptor-mediated internalisation
• Phagocytosis - uses PRRs
• Pinocytosis - cell drinking. DCs are constantly sampling the surroundings to detect pathogens
• Fc receptor mediated phagocytosis
• Complement receptor mediated phagocytosis

Question 10

How do cells go from endocytosis to presenting ag?

Answer 10

• Exogenous protein is endocytosed and exposed to proteases, NO, low pH and ROS in an endosome
• Will then become a late endosome/lysosome - becomes increasingly toxic
• Fuses with the MHC class II compartment
• MHC class II is exported from the ER and joins the compartment
• Invariant chain sits in the ag receptor for now, stabilising the molecule
• enzymes start to degrade invariant chain, leaving the CLIP peptide, which is then displaced by the ag
• HLA-DM helps to shuttle the loaded MHC to the surface, where it can present to CD4+ cells

Question 11

How does endogenous, cytoplasmic protein get processed?

Answer 11

• Cytoplasmic protein gets cleaved after going through the proteasome
• TAP is an ATP dependent transporter that shuttles the peptides into the ER
• Here it is loaded onto MHC I, which like MHC II, is unstable until bound Ag, so needs chaperones
• ER amino peptidases break the particles into smaller pieces in the ER
• When the MHC I is ready and loaded, it is taken up to the surface, through the golgi to the surface, and presented to CD8 T cells

Question 12

What are the differences in processing between endogenous and exogenous ags?

Answer 12

Ex

• Ag endocytosed
• Degraded in lysosomes
• Loaded onto MHC class II
• Presented to CD4+ Th cells

End

• Ag already in cell
• Degraded in proteasome
• Loaded onto MHC class I
• Presented to CD8+ CTL cells

Question 13

What happens with pathogens that dont infect APCs?

Answer 13

• e.g. avian flu only infects lung ciliated epithelia
• Epithelial cells arent very good at activating the immune system
• Have to carry out ag cross-presentation

Question 14

What is the process of ag cross-presentation?

Answer 14

• Endocytosed exogenous ags can go by cytosolic diversion to be fed into the proteasome, allowing MHC I production
• Ags are take to the proteasome, chopped up, fed into the ER and loaded on MHC I

Question 15

MHC class I vs Class II

Answer 15

Class I

• Expressed on all nucleated cells
• Binds short peptides
• Presents to CD8+ T cells
• Ags from cytosol + cross presentation
• Has 3 alpha subunits, a beta-2 microglobulin and a cytoplasmic tail

Class II

• Expressed on APCs, thymic epithelia and activated T-cells
• Binds long peptides
• Presents to CD4+ T-cells
• Ags from phagosomes and endosomes

Question 16

What are the 3 signals required for an APC to activate a T-cell?

Answer 16

• Ag presented on MHC complex
• Co-stimulatory molecules e.g. CD40L/CD40 or B7/CD28
• Cytokine release to dictate T cell lineage (Th1,2,17 or Treg)

Question 17

What cytokines dictate which T-cell type?

Answer 17

Th1 = IL-12

Th2 = IL-4

Th17 = IL-1b, IL-6, TGF-b, IL-23

Treg = IL-2, TGF-b

Question 18

What are the similarities between TCR and BCR?

Answer 18

• Belong to Ig superfamily
• Like Fab fragment of antibody
• Large diversity
• High specificity

Question 19

What are the differences in TCR compared to BCR?

Answer 19

• Lower affinity
• Cannot be released
• No Fc fragment
• Single rather than two binding sites
• BCR has 5 classes (IgM etc)
• TCR has 2 classes (ab and gd)

Question 20

What are the mechanisms that generate B/T-cell receptor diversity?

Answer 20

B cells

• Before Ag stim = somatic recombination
• After Ag stim = somatic hypermutation

T cells

• Before Ag stim = somatic recombination
• After = mone
• receptor gene rearrangement takes place during T-cell development in thymus

Question 21

What is the immunological synapse?

Answer 21

• The complex interaction of many molecules between T cell and APC
• MHC-TCR and Co-stimulatory signals are central
• There are lots of surrounding integrins and accessory molecules to helps stabilise the interaction

Question 22

What do the APC-T-cell reactions actually cause?

Answer 22

• Peptide-MHC binds to TCR -> Lck, Zap70 and PLC-g –> Calcium flux and NFAT activation -> T cell proliferation and survival
• Co-stimulation with CD28 -> PI3K, Akt, MAPK -> NF-kB activation -> pro-inflammatory survival signals

Question 23

How do T-cells induce changes in contacted cells?

Answer 23

• Activated CD8+ T cell can meet a virus-infected cell and secrete enzymes etc, killing the cell
• CD4+Th1 cells will meet macrophage. It will release cytokines such as IFN-gamma and activate the macrophage, allowing it to kill any invading bacteria
• CD4+Th2 cells will interact with a B cell and secrete cytokines that cause class switching. This B cell can differentiate into a plasma cell to secrete abs

Question 24

What are the negative regulators of antigen presentation?

Answer 24

• An overly-vigorous response is harmful to the host
• Negative regulators provide an immune checkpoint to limit T-cell activation
• CTLA-4 (cytotoxic-T-lymphocyte-Associate protein 4)
• PD-L1 (programmed death ligand 1)
• Both crucial for dampening the T-cell response
• TCR also down-regulates its own expression once recognised ag

Question 25

How do the negative regulators work?

Answer 25

• PD-L1 binds to PD-1, activating SHP-2, which dephosphorylates TCR signalling molecules
• CTLA-4 competes with CD28 for APC attention

Question 26

What are the different T-cell selection processes in the thymus?

Answer 26

• tested for self-reactivity
• If binds too strongly = apoptosed because dangerous (negative selection)
• If doesnt bind at all = apoptosed because wont protect against pathogens
• Positively selected if can bind but not too strongly
• In stochastic model, strongly self-reactive CD4+ T-cells can express FOXP3 TF and become Treg cells, and compete with any self-reactive T-cell

Question 27

How can pathogens impede antigen presentation?

Answer 27

• TB = upregulates PD-L1 and blocks MHC II expression
• n.meningitidis = blocks DC activation by downregulating receptors and MHC. Has capsule
• n.gonorrhoeae = expresse Opa protein, binds to T cells and induces tyrosine phosphatases, switching ogg TCR signalling molecules
• HIV - upregulates PD-L1 on T-cells and suppresses DC activation
• HSV = produce protein which inhibits TAP, prevents viral peptide transfer to ER
• Adenovirus = produce protein that binds to MHC class I preventing it from leaving ER